Observed Time Difference Of Arrival (OTDOA) is a downlink positioning method in LTE. OTDOA is a multilateration method in which a UE (user equipment) measures the time of arrival (TOA) of signals received from multiple base stations (Evolved Node Bs (eNBs)) and computes a reference signal time difference (RSTD) that is reported to the network. 3GPP (the Third Generation Partnership Project) defines OTDOA by using the Positioning Reference Signal (PRS).
Indoor UEs will experience more pathloss than outdoor UEs when eNBs are located outdoors. Thus, the number of detectable cells can be reduced for an indoor UE, as a result of the lower SINR (Signal to Interference-plus-Noise Ratio). Indoor positioning is currently being studied by 3GPP RAN (Radio Access Network) WG1 (working group 1) for Rel-13 (Release 13 of the 3GPP specification).
In 3GPP TS (technical specification) 36.133, describing E-UTRAN (evolved universal terrestrial RAN) OTDOA RSTD measurements, the UE physical layer can be capable of reporting RSTD for the reference cell with (PRS SINR)>=−6 dB and all the neighbor cells with (PRS SINR)>=−13 dB. These SINRs were set based on considerations involving outdoor UEs. For indoor UEs, the SINRs can be more stringent due to the signal having to penetrate the building.